Inductive magnetic recording heads (ferrite, metal-in-gap ferrite as well as thin-film heads) exhibit magnetic instability resulting from unstable and nonreproducible magnetic domain patterns in the yoke of the head after different write operations. These instabilities become more pronounced as track widths become narrower and the number of coil turns is increased to enhance recording density. One type of magnetic instability is "write instability"; i.e., the nonconstancy of the read response of a head following successive write operations as a result of Barkhausen noise. Another type of magnetic instability is "read instability" which can occur even when a head is not write-exercised for a long time and causes the data readback signal to vary or be distorted as a result of hysteretically moving domain walls. These types of instability can cause the head to respond to prerecorded magnetic transitions by affecting the readback signal in four different ways: (a) varying pulse amplitude, (b) varying pulse shape, (c) generating noise transients on predominantly the trailing edge of the pulse, and (d) generating noise transients randomly in time, predominantly directly after a write operation.
Phenomena (a) and (b) are caused by the domain patterns in predominantly the pole tip and apex regions of a thin-film head and in the crystallites adjacent to the gap and the airbearing surface of a ferrite head. Varying pulse amplitude (a) is due to variation in readback efficiency caused by variations in reluctance of the yoke associated with the different domain patterns following successive write operations. This results in a variation in the efficiency with which data signal flux is collected by the head. Varying pulse shape (b) is caused by the frequency dispersion in the part of the total data flux threading the coil that is contributed by predominantly reversible domain wall motion rather than by magnetization rotation. The different domain patterns occurring after write operations contribute differing fractions to the total flux sensed by the coil, resulting in differing dispersions; i.e., frequency dependencies. Phenomenon (c) results from the irreversible part of the wall motion induced by the data signal, which produces noise transients in head output voltage that are correlated to the readback signal. Finally, effect (d) results from delayed relaxation of the domain pattern in the head yoke after a write operation.
As a result of these phenomena (1) equalization of the channel becomes ambiguous because there is a plurality of pulse shapes to be equalized, rather than a single, isolated pulse shape; (2) the noise transients can create peak shift or extra peaks in a peak-detect channel and cause signal-correlated noise in a partial response maximum likelihood (PRML) channel; (3) specific domain patterns in the yoke of the head may produce enough remanence of the pole tips to partially erase the magnetic data transitions on the recording disk because even a weak field will reduce the magnetic data after many passes; and (4) servo sector information may become erratic (i.e., produce jittery position error signals) when writing data in a disk drive with embedded servo. Magnetic head instability, therefore, limits the use of inductive heads in high linear density applications using narrow track widths, and/or using embedded servo.
U.S. Pat. No. 4,280,158 discloses a magnetoresistive (MR) head comprising an MR sensor connected to an amplifier for suppressing Barkhausen noise by generating a negative feedback magnetic field that produces a magnetic flux in the MR sensor of a polarity opposite that of the field to be detected. The MR sensor is DC-biased, and a DC sense current is needed for readout of the sensor. A separate actuator is required in the form of a coil or "turn" to generate the field. The negative feedback is generated by a loop circuit. The purpose of the MR head is to linearize the resistance of the MR sensor in relation to the magnetic data signal input.
No prior art known to applicants discloses circuitry for eliminating magnetic instability of an inductive magnetic head by inhibiting motion of the domain walls in the yoke of the head during sensing of data so that inductive magnetic heads can be used in applications requiring narrow track widths, high linear densities, and embedded servo.